The invention relates to mixtures of isomeric nonanols and decanols, a process for their preparation, the phthalic esters obtained from these alcohols mixtures, and the use of these esters as plasticizers.
Esters of phthalic acid have wide application as plasticizers, in particular for polyvinyl chloride. The alcohol components are principally primary alcohols having from 8 to 10 carbon atoms, the most important among them presently being 2-ethylhexanol. Phthalic esters of short-chain alcohols give plasticizers with good gelling powder; however, their higher volatility is a disadvantage. In comparison, long-chain esters gel more slowly but have poorer cold resistance.
The properties of the phthalic ester plasticizers are affected, not only by the size of the alcohol molecule, but also by the branching of the hydrocarbon chain. Thus, alcohols with little branching give ester plasticizers of high cold flexibility. Largely linear alcohols having from 9 to 10 carbon atoms in the molecule are therefore becoming increasingly important as alcohol components. A prerequisite for their use is that they are available in large quantities and at advantageous prices.
In German Patent 28 55 421, the plasticizers used are phthalates of C9-alcohols, which are obtained by the oxo-reaction of C-8-olefins, hydrogenation of the reaction product, and esterification of the C9-alcohols with phthalic anhydride. From 3% to 20% by weight of the starting olefins is said to have an isobutane skeleton in each molecular chain, less than 3% by weight of the olefins should contain quaternary carbon, and more than 90% by weight of the total amount of olefins is said to be present as n-octenes, monomethylheptenes, and dimethylhexenes. Furthermore, the weight ratio of the total amount of the n-octenes and monomethylheptenes to the dimethylhexenes is said to be more than 0.8.
Phthalic esters based on C10-alcohols are the subject of the European Patent Application 3 66 089. The C10-alcohols are used in the form of a mixture which is obtained by hydroformylation of a butene fraction, aldol condensation of the aldehyde mixture thus obtained, and subsequent hydrogenation. According to the process description, the hydroformylation step is not subject to any limitations. The catalysts used may be cobalt as well as rhodium; the addition of an organic compound of trivalent phosphorus is not excluded.
Another route to obtaining didecylphthalate mixtures is described in European Patent Application 4 24 767. The preparation of the esters is carried out in a multistage process by dimerization of butene mixtures, hydroformylation and hydrogenation of the resulting octene mixture to give a nonanol mixture, dehydration of the nonanol mixture to form a nonene mixture, and hydroformylation and hydrogenation of the nonene mixture to form the desired decanol mixture.
According to EP-B-52 999, plasticizer alcohols are prepared from a mixture of propylene and butenes in a molar ratio of 2:1 to 1:3. The olefins are jointly converted by the oxo reaction to a mixture of butyl and amyl aldehydes which is subjected to an aldol condensation. The resulting condensation products are subsequently hydrogenated to saturated alcohols.
The known alcohols or alcohol mixtures used for the preparation of plasticizers do not meet all the economic and technical requirements which are demanded of products produced on an industrial scale, because the starting materials are not available in sufficient quantity, the prices are too high, the conversion of the starting materials into the alcohols necessitates extremely costly processes, and/or the quality of the plasticizers prepared from the alcohols leaves much to be desired.
It is therefore an object of the present invention to develop suitable alcohol or alcohol mixtures for the preparation of high-quality plasticizers. They should be obtained from economically available raw materials in a technically simple manner.
This object is achieved by mixtures of isomeric nonanols and decanols which are obtained by joint aldol condensation of n-butanal and pentanals in a molar ratio of from 1:2 to 1:10. The pentanal mixtures comprise 60% to 90% by weight of n-pentanal, 10% to 40% by weight of 2-methylbutanal and up to 1% by weight of 3-methylbutanal. The aldol condensation product is then hydrogenated to form the saturated alcohols, and the components boiling at lower temperatures than the nonanols and decanols are removed from the reaction mixture.
It is preferable to use mixtures of isomeric nonanols and decanols which are prepared from n-butanal and pentanals which contain from 65% to 80% by weight of n-pentanal, 20% to 35% by eight of 2-methylbutanal, and up to 1% by weight of 3-methylbutanal. The alcohol mixtures are obtained by aldol condensation of a mixture containing n-butanal and pentanals in a molar ratio of 1:2 to 1:10, subsequent hydrogenation of the aldol condensation product, and removal of the 2-ethylhexanol formed. The source of the aldehydes is immaterial; the criteria are chiefly economic. To promote the formation of alcohols with little branching, the aldehydes must have the carbonyl group on the terminal carbon atom and, in the case of the pentanals, be at least substantially unbranched. Therefore, the pentanals used are mixtures containing from 60% to 90% by weight of n-pentanal, from 10 to 40% by weight of 2-methylbutanal, and up to 1% by weight of 3-methylbutanal.
Preferred starting materials are aldehydes prepared by hydroformylation (oxo process) of propylene or butenes. The required olefins are available in industrial quantities. Propylene is obtained as byproduct in ethylene production by pyrolysis of hydrocarbon mixtures in the presence of water vapor and also in some refinery processes, particularly the catalytic cracking of petroleum fractions.
Mixtures containing butene-1 and butene-2 are also necessarily obtained in considerable quantities as refinery byproducts in the production of automotive fuels and in the production of ethylene by thermal cracking of higher hydrocarbons. They are isolated from the C4 cracking fractions of the pyrolysis product by extraction of the butadiene-1,3 by a selective solvent, and subsequent removal of the isobutene preferably by conversion into methyl t-butyl ether. Instead of extracting the butadiene-1,3, it can also be partly hydrogenated to butenes in the C4 cracking fraction. The pyrolysis product freed of butadiene-1,3 is identified as raffinate I. If the isobutene has also been removed, it is referred to as raffinate II. This butene-1/butene-2 mixture is particularly suitable for further processing into decanols.
Basically, all current commercial hydroformylation processes are suitable for converting the olefins into aldehydes. Thus, the process can be carried out in the presence of cobalt or rhodium catalysts at pressures of 10 to 35 MPa and at temperatures of 120xc2x0 to 180xc2x0 C.; in the presence of cobalt/phosphine catalysts at pressures of from 5 to 10 MPa; or in the presence of rhodium catalysts which are modified by phosphine at temperatures of 60xc2x0 to 150xc2x0 C. and pressures of 1 to 8 MPa. In the last-described variant of the hydroformylation reaction, the catalyst may be homogeneously dissolved inxe2x80x94or form a separate phase fromxe2x80x94the reaction mixture.
To prepare the aldehydes, propylene and the butenes may be reacted together, but preferably separately. It has proven particularly valuable to carry out the hydroformylation as a heterogeneous reaction in a two-phase system, a reaction which is described, for example, in DE-C-26 27 354. This embodiment of the oxo process ensures that olefins having their double bonds at a terminal carbon atom form largely n-aldehydes and that isomerization of the olefins by migration of the double bond during the reaction is essentially avoided.
The two-phase process is characterized by the presence of an organic phase, which contains the starting olefins and the reaction product, and an aqueous phase, in which the catalyst is dissolved. Catalysts used are water-soluble rhodium complexes which contain water-soluble phosphines as ligands. The phosphines include, in particular, triarylphosphines, trialkylphosphines, and arylated or alkylated diphosphines, the organic radicals of which are substituted by sulfonic acid groups or carboxyl groups. Their preparation is known and described, for example, in DE-PS 26 27 354 and DD-PS 259 194. The reaction of the olefins is carried out at temperatures of 70xc2x0 to 150xc2x0 C., preferably 100xc2x0 to 130xc2x0 C., and at pressures in the range of 0.4 to 30, in particular 1 to 10, MPa; the water gas used contains carbon monoxide and hydrogen in a volume ratio of 1:10 to 10:1. The rhodium concentration is 20 to 1000 ppm by weight, preferably 50 to 500 ppm by weight, based on the aqueous catalyst solution, with from 4 to 100 mol of water-soluble phosphine being used per mole of rhodium. The volume ratio of aqueous to organic phase is from 0.1 to 10:1.
The conversion of the butenes is appreciably increased if a phase-transfer reagent (solubilizer) is added to the aqueous catalyst solution. Materials which have proven particularly valuable are cationic solubilizers of the formula [Axe2x80x94N(R1R2R3)]+E, wherein A is a straight or branched chain alkyl radical having 6 to 25 carbon atoms; R1, R2, R3 are individually straight or branched chain alkyl radicals having from 1 to 4 carbon atoms; and E is for example sulfate, tetrafluoroborate, acetate, methosulfate, benzenesulfonate, alkylbenzenesulfonate, toluenesulfonate, lactate, or citrate.
In the described process, as much as 99% of the propylene is converted, the butanal mixture obtained comprising over 95% by weight of the n-compound. When butene-1/butene-2 mixtures are used, the reaction with butene-1 is preferred. Depending on the reaction parameters selected, more than 95% of the butene-1 or butene-2 is converted. From 60% to 90% by weight of n-pentanal is formed, the remainder comprising 2-methylbutanol with or without 3-methylbutanal.
After completion of the separate or joint hydroformylation, the aldehydes are separated from the catalyst, from the unreacted reaction components, and from the other reaction products. In the case of the heterogeneous reaction, this is by simple phase seperation. For reaction in the homogeneous phase, a usual separation process such as distillation suffices.
In the subsequent aldol condensation, mixtures are used which contain, per mole of n-butanal, 2 to 10 mol, in particular 7 to 10 mol, of pentanals. The reaction of the aldehyde mixture is carried out in the conventional way using basic catalysts. Pretreatment of the aldehydes, for example a special purification, is not necessary. It is, however, advisable in the case of the butanals to remove i-butanal from the C4-aldehyde mixture by distillation, if the proportion thereof in the mixture exceeds approximately 2% by weight. Suitable catalysts are alkali metal carbonates or alkali metal hydroxides, in particular compounds of sodium or potassium and amines, preferably tertiary amines, such as triethylamine, tri-n-propylamine and tri-n-butylamine. The reaction is carried out at temperatures of 60xc2x0 to 160xc2x0 C., in particular 80xc2x0 to 130xc2x0 C., and at atmospheric pressure or at a superatmospheric pressure of up to 1 MPa. The reaction time is from a few minutes to several hours and is, in particular, dependent on the catalyst type and reaction temperature. Because of their higher reactivity, the straight-chain aldehydes react preferentially. Self-condensation of n-butanal or n-pentanal forms C8 or C10-enals and the mixed condensation of n-butanal and n-pentanal gives C9-enals. The reactions between n-butanal or n-pentanal and branched-chain pentanals proceed at appreciably lower rates; the reaction between branched-chain pentanals is largely insignificant.
The mixture of unsaturated aldehydes obtained by condensation is subsequently hydrogenated to a mixture containing nonyl and decyl alcohols together with 2-ethylhexanol and any butanols and pentanols arising from C4- and C5-aldehydes which may not have been converted by the aldol condensation. The addition of hydrogen is carried out in a known manner in the presence of catalysts. Suitable catalysts are, for example, hydrogenation catalysts based on nickel, chromium or copper. The hydrogenation temperature is usually between 100xc2x0 and 180xc2x0 C. and the pressure is 1 to 10 MPa. According to the invention, the alcohol mixture obtained is subjected to distillation at 100xc2x0 to 125xc2x0 C. and a pressure of 1 to 4 kPa (from 10 to 40 mbar) to remove 2-ethylhexanol, other alcohols, and impurities which boil at lower temperatures than the nonanols and decanols.
The remaining mixture of nonanols and decanols is especially suitable as the alcohol component in phthalic esters which are to be used as plasticizers. The preparation of phthalic esters is known [cf. Ullmann, Encyclopadie der Technischen Chemie (1979), Vol. 18, page 536 ff]. Phthalic anhydride is advantageously reacted with the nonanol/decanol mixture in a molar ratio of 1:2 to 1:3 in a single stage. The reaction rate can be increased by catalysts and/or by increasing the reaction temperature. To shift the equilibrium in the direction of ester formation, it is necessary to remove the water of reaction from the reaction mixture.
The phthalates obtained from the nonanol/decanol mixture of the invention are remarkable for their low volatility and good gelling ability.